A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern may be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the lithographic apparatus as hereabove specified, an article to be placed in the radiation beam may be held to an article holder by a clamping electrode. Such electrostatic clamping may be used, for example, when a substrate is processed in vacuum conditions. This type of processing occurs, for example, when the type of irradiation used for photolithographic processes is in the (soft) x-ray region, also referred to as Extreme Ultraviolet (EUV) region. Electrostatic clamping makes use of an electric field between the article holder and the article. In this way, electrostatic forces may be used that are present between electrical charge differences in (local) areas of the article holder and the article in order to provide a clamping pressure. Such electrical charge differences occur when the electrode in the article holder is charged and, for example, the article is grounded. Otherwise, in the article holder, a plurality of electrodes of opposite voltages may be present, which introduce an opposite charge distribution in the article, preferably leaving no excess charge in the article. In a setting where such a multipolar clamp is used, there is a risk that the article may be unevenly clamped, which may result in deformation of the article, and a corresponding loss in image quality when used for photolithographic purposes. Ultimately, such uneven clamping may even result in handling problems during placement and removal of the article, which may cause loss of production time and may even cause loss of the article.
In the context of this application, the “article” may be e.g., a wafer, reticle, mask, or substrate.